Electrons in AtomsElectrons in Atoms

Models of the AtomModels of the Atom• Atoms are the smallest indivisible part of an

element—who stated this?• John Dalton (thanks to Democritus)

• The atom is a ball of positive charge with electrons spread throughout—who stated this?

• JJ Thomson

• An atom’s mass is concentrated in the small positively charged nucleus. The electrons surround the nucleus and the rest of the atom is empty space—who stated this?

• Rutherford

• Electrons are arranged in concentric paths around the nucleus—who stated this?

• Neils Bohr

Ernest Rutherford’s Ernest Rutherford’s ModelModel

• Discovered dense positive piece at the center of the atom- “nucleus”

• Electrons would surround and move around it, like planets around the sun

• Atom is mostly empty space• It did not explain the

chemical properties of the elements – a better description of the electron behavior was needed

Niels Bohr’s ModelNiels Bohr’s Model• Why don’t the electrons fall into

the nucleus?• Move like planets around the

sun.•In specific circular paths, or orbits,

at different levels.•An amount of fixed energy

separates one level from another.

The Bohr Model of the The Bohr Model of the AtomAtom

Niels Bohr

I pictured the electrons orbiting the nucleus much like planets orbiting the sun.

However, electrons are found in specific circular paths around the nucleus, and can jump from one level to another.

Bohr’s modelBohr’s model

• Energy level of an electron: region around the nucleus where an electron is likely to be moving• analogous to the rungs

of a ladder• The electron cannot exist

between energy levels, just like you can’t stand between rungs on a ladder

• A quantum of energy is the amount of energy required to move an electron from one energy level to another

• The higher the electron is on the “energy ladder” the farther it is from the nucleus.

The Quantum Mechanical The Quantum Mechanical ModelModel

• The quantum mechanical model of the atom estimates the probability of finding an electron in a certain location

• Energy is quantized - It comes in chunks.• A quantum is the amount of energy

needed to move an electron from one energy level to another.

• In 1926, Erwin Schrodinger derived an equation that described the energy and position of the electrons in an atom

Schrodinger’s Wave Schrodinger’s Wave EquationEquation

22

2 2

8dh EV

m dx

This Equation gives the probabilityprobability of a single electron being found along a single axis (x-axis)Erwin SchrodingerErwin Schrodinger

• Things that are very small behave differently from things big enough to see.

• The quantum mechanical model is a mathematical solution

• It is not like anything you can see.

The Quantum The Quantum Mechanical ModelMechanical Model

• There are energy levels for electrons.

• Orbits are not circular.• We can only know the probability

of finding an electron a certain distance from the nucleus.

The Quantum The Quantum Mechanical Model Mechanical Model says…says…

• The atom is found inside a blurry “electron cloud”

• An area where there is a chance of finding an electron.

• Think of fan blades

The Quantum The Quantum Mechanical ModelMechanical Model

The physics of the very The physics of the very smallsmall

•Quantum mechanics explains how very small particles behave•Quantum mechanics is an

explanation for subatomic particles and atoms as waves

•Classical mechanics describes the motions of bodies much larger than atoms

Atomic OrbitalsAtomic Orbitals• Principal Quantum Number (n) = the

energy level of the electron: 1, 2, 3, etc.

• Do you remember how many there are?

• Within each energy level, there are sublevels

• The complex math of Schrodinger’s equation describes several shapes of these sublevels

• These “shapes” are called atomic orbitals - regions where there is a high probability of finding an electron

Principal Quantum Principal Quantum NumberNumber

Generally symbolized by “n”, it denotes the shell (energy level) in which the electron is located.

Maximum number of electrons that can fit in an energy level:

2n2

SummarySummary

s

p

d

f

# of shapes

Max electrons

Starts at energy level

1 2 1

3 6 2

5 10 3

7 14 4

By Energy LevelBy Energy Level

• First Energy Level

• Has only s orbital

• only 2 electrons• 1s2

• Notice 2(1)2

• Second Energy Level

• Has s and p orbitals available

• 2 in s, 6 in p

• 2s22p6

• 8 total electrons• Notice 2(2)2

By Energy LevelBy Energy Level

• Third energy level• Has s, p, and d

orbitals• 2 in s, 6 in p, and

10 in d

• 3s23p63d10

• 18 total electrons• Again 2(3)2

• Fourth energy level• Has s, p, d, and f

orbitals• 2 in s, 6 in p, 10 in

d, and 14 in f

• 4s24p64d104f14

• 32 total electrons• 2(4)2

By Energy LevelBy Energy Level

Any more than the fourth and not all the orbitals will fill up.

• You simply run out of electrons

• The orbitals do not fill up in a neat order.

• The energy levels overlap

• Lowest energy fill first.

Electron ConfigurationElectron Configuration

• Let’s review…• Give the electron for Sodium (Na)• 1s22s22p63s1

• Remember the periodic table…

s block

p block

d block

f block

Periodic table and Periodic table and orbitalsorbitals

Light: backing up a bit…Light: backing up a bit…• The study of light led to the development

of the quantum mechanical model.• Light is a kind of electromagnetic radiation.• Electromagnetic radiation includes many

types: gamma rays, x-rays, radio waves… • Speed of light = 2.998 x 108 m/s, and is

abbreviated “c”• All electromagnetic radiation travels at this

same rate when measured in a vacuum

“R O Y G B I V”

Frequency Increases

Wavelength Longer

Parts of a waveParts of a wave

Wavelength

AmplitudeOrigin

Crest

Trough

Equation:

c =

c = speed of light, a constant (2.998 x 108 m/s)

(nu) = frequency, in units of hertz (hz or sec-1) (lambda) = wavelength, in meters

Electromagnetic radiation Electromagnetic radiation propagates through space as a propagates through space as a wave moving at the speed of light.wave moving at the speed of light.

Wavelength and Wavelength and FrequencyFrequency

• Are inversely related• As one goes up the other goes down.

• Different frequencies of light are different colors of light.

• There is a wide variety of frequencies

• The whole range is called a spectrum

Radiowaves

Microwaves

Infrared .

Ultra-violet

X-Rays

GammaRays

Low Frequency

High Frequency

Long Wavelength

Short WavelengthVisible Light

Low Energy

High Energy

Long Wavelength

=Low Frequency

=Low ENERGY

Short Wavelength

=High Frequency

=High ENERGY

Calculating wavelengthCalculating wavelength

• Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44 x 109 Hz?

Atomic SpectraAtomic Spectra

• White light is made up of all the colors of the visible spectrum.

• Passing it through a prism separates it.

If the light is not whiteIf the light is not white

• By heating a gas with electricity we can get it to give off colors.

• Passing this light through a prism does something different.

Atomic SpectrumAtomic Spectrum

• Each element gives off its own characteristic colors.

• Can be used to identify the atom.

• This is how we know what stars are made of.

• These are called the atomic emission spectrum

• Unique to each element, like fingerprints!

• Very useful for identifying elements

Explanation of atomic Explanation of atomic spectraspectra

• When we write electron configurations, we are writing the lowest energy.

• The energy level, and where the electron starts from, is called it’s ground state - the lowest energy level.

Changing the energyChanging the energy• Let’s look at a hydrogen atom,

with only one electron, and in the first energy level.

Changing the energy• Heat, electricity, or light can move

the electron up to different energy levels. The electron is now said to be “excited”

Changing the energy• As the electron falls back to the

ground state, it gives the energy back as light

• electrons may fall down in specific steps

• Each step has a different energy

Changing the energy

{{{

• The further they fall, more energy is released and the higher the frequency.

• This is a simplified explanation!• The orbitals also have different

energies inside energy levels• All the electrons can move around.

Ultraviolet Visible Infrared

Calculating the energy of Calculating the energy of a quantum or photona quantum or photon• E=h• Energy of a photon = Planck’s constant

x frequency• Planck’s constant = 6.626 x 10-34 J-s• Every object gets its color by reflecting a certain

portion of incident light. The color is determined by the wavelength of the reflected photons, thus by their energy. What is the energy of a photon from the violet portion of the Sun’s light if it has a frequency of 7.230 x 10 14 Hz?

Wave-Particle DualityWave-Particle DualityJ.J. Thomson won the Nobel prize for describing the electron as a particle.

His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron.

The electron is a particle!

The electron is an energy

wave!

Heisenberg Heisenberg Uncertainty PrincipleUncertainty Principle

• It is impossible to know exactly the location and velocity of a particle.

• The better we know one, the less we know the other.

• Measuring changes the properties.• True in quantum mechanics, but not

classical mechanics

Heisenberg Uncertainty Heisenberg Uncertainty PrinciplePrinciple

You can find out where the electron is, but not where it is going.

OR…

You can find out where the electron is going, but not where it is!

“One cannot simultaneously determine both the position and momentum of an electron.”

Werner Heisenberg

It is more obvious with the It is more obvious with the very small objectsvery small objects•To measure where a

electron is, we use light.•But the light energy moves

the electron•And hitting the electron

changes the frequency (or velocity) of the light.

Moving Electron

Photon

Before

Electron velocity changes

Photon wavelengthchanges

After